This invention relates to a fuel assembly for a light water reactor, and more particularly to a fuel assembly having a water rod suitable for reducing a pressure loss and improving the core reactivity gain.
A boiling water reactor is provided with a plurality of fuel assemblies in a core thereof so that the fuel assemblies are spaced from one another. Each fuel assembly consists of a plurality of fuel rods, water rods, upper and lower tie plates, spacers and a channel box.
The water rods are provided with holes in the upper and lower portions of their side surfaces, and a coolant is passed through the interior of each of the water rods via these holes.
In the interior of the fuel assembly, the coolant of water flows to remove the thermal energy which occurs due to a reaction of a fission substance existing in the fuel rods. The water also flows in the spaces among the fuel assemblies, i.e. the exterior of the channel box.
The water functions also as a moderator. The fast neutrons flying out into the water turn into moderated thermal neutrons due to the water. When such thermal neutrons collide with a fission substance, a fission reaction occurs. However, voids occur due to the heat generated by the fuel rods in the channel box. The spaces among the fuel rods are narrower than those among the fuel assemblies, and the former contains more water than the latter. Accordingly, the local atomic ratio of water to fuel in the central portion of the fuel assembly becomes smaller than that in the peripheral portion thereof. As a result, it is difficult for the fast neutrons be moderated in the central portion of the fuel assembly, so that the thermal neutron flux therein tends to become low. This causes the fuel rod power distribution in the fuel assembly to become unbalanced, and has a disadvantageous effect on the economy of fuel. In order to solve this problem, a moderator-container or the water rod is provided in the fuel assembly, which is disclosed in, for example, Japanese Patent Laid-open No. 40986/1975. In recent years, a method of increasing the exposure of a fuel for the purpose of improving the economical efficiency of a nuclear power plant, and of increasing the the diameter of a water rod so as to improve the water to fuel atomic ratio and thereby cope with the increase in the enrichment of fuel has been studied.
Providing a large-diameter water rod or a container, in which a moderator, such as water is held, in the central portion of a fuel assembly is disclosed in Japanese Patent Laid-open Nos. 40986/1975 and 13981/1984. It is known that, owing to such a water rod or container, the distribution of thermal neutron flux in a fuel assembly becomes uniform to cause the simplification of the enrichment distribution and an increase in the reactivity, which enable the economical efficiency of the fuel to be improved.
However, if the diameter of the water rod is increased, the cross-sectional area of the flow passage for a coolant in the fuel assembly decreases. This causes an increase in the pressure loss and a decrease in the thermal margin. Therefore, it is necessary that a suitable countermeasure be provided.